1. Field of the Invention
The present invention relates to a diagnostic system for a variable valve timing control system of automotive internal combustion engines, and specifically to a system for diagnosing whether a variable valve timing control system itself operates in a normal state or in an abnormal state.
2. Description of the Prior Art
In modem vehicles, a variable intake- and/or exhaust-valve timing control system is often provided for variably adjusting an intake valve timing and/or an exhaust valve timing so that the optimum performance can be obtained corresponding to the use of the engine or vehicle. In such a variable valve timing control system for automotive engines, an intake- and/or exhaust-valve timing would be varied depending on engine revolutions, engine load and the like. One such valve timing control system has been disclosed in Japanese Patent Provisional Publication No. 2-298615 or in U.S. Pat. No. 4,231,330. As may be appreciated, in the event that the variable valve timing control system malfunctions, the optimum performance cannot be obtained. For the reasons set out above, there have been developed and proposed various diagnostic systems for a variable valve timing control system of automotive engines. In most popular diagnostic systems for a variable valve timing control system, for the purpose of diagnosing the normality or abnormality of the valve timing control system, the diagnostic system forcibly switches from an ON position (an operative state) of the valve timing control system to an OFF position (an inoperative state) under diagnostic permissible conditions for example a predetermined engine revolution range, a predetermined intake-air flow rate or the like, and then the diagnostic system monitors variations in the operating state of the engine (such as a rate of change in intake-air flow), resulting from the forcible switching operation to the OFF position. After the forcible switching operation, the diagnostic system determines that the valve timing control system operates normally when the variation of the operating state of the engine exceeds a predetermined reference value. In contrast to the above, when the variation of the operating state of the engine is less than the reference value, the diagnostic system determines that the valve timing control system operates abnormally.
Irrespective of whether or not the valve timing control system is shifted from the operative state to the inoperative state or vice versa, there are comparatively great positive and negative fluctuations in the rate of intake-air flow during acceleration or during deceleration. In such a case, it is difficult to precisely diagnose the valve timing control system. In the case that the engine experiences great fluctuations in the rate of intake-air flow owing to acceleration or deceleration, it is necessary to inhibit the incorrect diagnosis on the valve timing control system. To avoid the incorrect diagnosis, it is advantageous to properly inhibit the diagnosis on the valve timing control system, depending on a rate of change in the throttle valve opening. For example, it is preferable to inhibit the diagnosis on the valve timing control system when the rate of change in throttle valve opening is greater than a predetermined threshold. The magnitude relationship between the rate of change in throttle valve opening and the predetermined threshold is dependent on a sampling time interval of the throttle valve opening. Assuming that the throttle valve opening is detected every predetermined sampling time intervals, such as 0.1 sec (a comparatively short time interval) and additionally the rate of change in throttle valve opening is less than the predetermined threshold at the above-noted short time intervals, the system determines that it is unnecessary to inhibit the diagnosis because of a less rate of change in throttle valve opening. Thereafter, the diagnosis will be made on the valve timing control system. However, even when the rate of change in throttle valve opening is less than the predetermined threshold at the above-noted short time intervals, there is a possibility that the rate of change in throttle valve opening may become greater than the predetermined threshold in the event that the sampling time interval of the throttle valve opening is set at a comparatively long interval such as 1 sec. In this case, the diagnosis cannot be inhibited properly and timely, and thus there is a possibility that the diagnostic system may make an incorrect diagnosis on the valve timing control system. If the predetermined threshold of the rate of change in throttle valve opening is set at a smaller value, the system may operate to more frequently inhibit the diagnosis on the valve timing control system owing to slight fluctuations in the throttle valve opening. This undesiredly reduces the frequency of the diagnosis on the variable valve timing control system. Alternatively, assuming that the sampling time interval of the throttle valve opening is set at a longer time interval such as 2 sec, the diagnostic system begins to make a diagnosis on the valve timing control system with a time lag of 2 sec from the time when the rate of change in the throttle valve opening has been reduced down to a value less than the predetermined threshold. This also reduces the frequency of the diagnosis.